Question

A series circuit has a battery and resistor and a light bulb with a power of 3.0 W if the current in the circuit is 0.90 A over a period of 20 seconds what is the voltage across a lightbulb

Answer 1

Power = (current)² x (resistance)

Resistance = (power) / (current)²

Resistance = (3 W) / (0.9 A)²

Resistance = 3.333... ohms

That's the total resistance in the series circuit. (3 and 1/3) ohms .

Voltage = (current) x (resistance)

Voltage = (0.9 A) x (3.333... ohms)

Voltage = 30 volts

That's the total voltage ... the battery that's powering the whole series circuit.

Using only the information given in the question, there's no way to calculate how much of the 30V appears across the resistor and how much of it appears across the light bulb.